The Keketale is the largest Pb–Zn deposit in the volcano‐sedimentary Maizi Basin of the South Altay Orogenic Belt (AOB), Northwest China. The stratabound orebodies are hosted in a suite of meta‐sedimentary rocks intercalated with volcanic rocks of the Lower Devonian Kangbutiebao Formation. The massive and banded ores representing the main mineralization stage are relatively well‐preserved in the ore district. This paper reports systematic geochronological results including the zircon laser ablation–multiple collector–inductively coupled plasma–mass spectrometry (LA‐MC‐ICP‐MS) U–Pb analyses on two meta‐felsic volcanic rocks from the Kangbutiebao Formation and Rb–Sr isotope dating on seven sphalerite samples from the main mineralization stage, together with some sulphur isotopic data to constrain the mineralization age and the genesis of the deposit. Rb–Sr isotope dating yield an isochron age of 398.2 ± 3.3 Ma generally synchronous with the zircon (LA‐MC‐ICP‐MS) U–Pb analyses of a meta‐rhyolite and a meta‐dacite from the strata (410.5 ± 1.3 Ma and 394.8 ± 1.9 Ma, respectively). The δ 34 S values of seven pyrite samples in the main massive and banded ores vary from −12.4‰ to −6.2‰, indicating that the main ore‐forming sulphur of the deposit was derived from bacterial reduction of seawater sulphate. By integrating the field, chronological, and isotopic evidences, we conclude that the Keketale Pb–Zn deposit is a VMS‐type deposit. Combining our results with the isotopic geochronology in the South AOB, we argue that the South AOB has undergone three mineralization episodes: the syndepositional mineralization (412–387 Ma), the subvolcanic hydrothermal‐related mineralization (382–379 Ma), and the epigenetic mineralization that is genetically linked to regional metamorphism and deformation (260–204 Ma). The Keketale Pb–Zn deposit is a product of the Devonian seafloor hydrothermal exhalation system in the South AOB.
Scheelite is the main ore mineral in skarn-type tungsten (W-Mo, W-Sn, and W-Cu) deposits, and is also a good proxy for ore-fluid evolution and mineralization. The Bastielieke deposit is the first medium-size W-polymetallic skarn deposit discovered in the Chinese (Xinjiang, NW China) Altay. Scheelite grains at Bastielieke are distributed in biotite granite, skarn and quartz-fluorite veins. They exhibit different textures, and can be divided into four types and six subtypes, including those in granite (Schm), prograde skarn (SchI), retrograde-altered rocks (SchII), and those in late-stage quartz-fluorite veins (SchIII). SchIa and SchIb were formed in the early and late prograde stage, respectively. SchI displays homogeneous texture, enrichments of light rare earth elements (LREEs) relative to heavy REEs (HREEs), and significantly negative Eu anomalies. SchIa has higher Sr-Mo contents and LREE/HREE than SchIb. SchII shows patchy texture by overgrowth and dissolution-reprecipitation, and can be subdivided into dark (SchIIa) and light (SchIIb) zone based in CL imaging. All SchII grains are LREE-enriched with negative Eu anomalies and relatively low LREE/HREE ratios. SchIIb has much higher W-Mo-Nb-Sr contents than SchIIa, which is ascribed to late-stage hydrothermal modifications. Schm and SchIII display homogeneous texture and similar MREE-enriched patterns, as well as very low Mo-W-Sr and different Nb contents. The texture and compositional variations in Bastielieke scheelites reveal that two magmatic fluids derived from different granitic magma reservoirs were involved in the mineralization. The earlier ore-fluid is relativly oxidized and has low HREE contents, forming the early prograde skarn-stage mineralization. Episodic influxes of later F-rich granitic magmatic fluids may have modified the earlier scheelite compositions, leading to multistage W enrichment and varying intragrain compositions.
The Chonghuer basin is one of the fault-bounded volcano-sedimentary basins on the southern margin of the Altay orogenic belt, where the Kangbutiebao Formation (Fm.) stratigraphic unithosted many economic deposits outcrops. LA-ICP-MS U-Pb zircon analyses from two meta-rhyolites and one meta-tuff of the Kangbutiebao Fm. in the Chonghuer Basin yield weighted mean 206Pb/238U ages of 385.3±1.2Ma,398.1±1.8Ma and 405.6±2.2Ma, respectively, which can be interpreted as the eruption age of the Kangbutiebao silicic volcanic rocks in the Chonghuer Basin. The meta-rhyolites have similar geochemical features to meta-rhyolites in Kelang Basin and Maizi Basin, which are low-Ti rhyolite with obvious negative anomaly of Ti, P, Sr, Ba, Nb, Ta and Eu (δEu=0.25~0.54), obvious enrichment of Th, U, Pb, Zr, Hf and LREE. These features, together with the regional geological characteristics, indicate that these felsic volcanic rocks were generated by partial melting of lower crust on an active continental margin. These volcano-sedimentary basins are down-faulted Basin formed in compression tectonic environment of Early—Middle Devonian period.
Extensive Permian mafic–ultramafic intrusions crop out within the eastern Tianshan, southern part of Central Asian Orogenic Belt (CAOB). Most of these mafic–ultramafic complexes are associated with Cu-Ni-Co deposits. However, Cihai, located in the southern part of the eastern Tianshan, is a large Fe deposit hosted in the Early Permian mafic rocks. The mafic to intermediate rocks are composed of gabbro, diabase, and monzodiorite. Geological and geochemical characteristics suggest that their parental magmas might have been generated by interaction between the depleted asthenospheric mantle and the metasomatized lithospheric mantle. Iron ores of the Cihai iron deposit are hosted in the diabase, and all Fe–Ti oxides in the ore-hosted diabase are ilmenite, instead of magnetite as previously reported. Chondrite-normalized REE patterns show that the magnetite separates from disseminated, banded, and massive iron ores, which are distinct from those in magmatic Fe-Ti deposits. Geological and chemical features suggest that the main ore bodies in the Cihai iron deposit are of hydrothermal origin, rather than magmatic as previously suggested. Numerous other Early Permian mafic rocks were recently identified in the Tarim basin and the eastern Tianshan with ages between 301 and 269 Ma. The mafic rocks in the Tarim basin exhibit characteristics of Oceanic Island Basalt (OIB), whereas the mafic rocks in the eastern Tianshan show island arc basalt (IAB) affinity. In addition, the presence of skarn iron deposit instead of Fe–Ti oxide deposit in the eastern Tianshan during the Early Permian time also lends little support for a plume-related environment. These features, together with a lack of verified anomalous high-temperature magmas in the eastern Tianshan, suggest that the Permian Tarim mantle plume may not account for the formation of the mafic rocks in the eastern Tianshan area, and that the Tarim LIP does not extend to the eastern Tianshan area.
Extensive Permian mafic–ultramafic intrusions crop out within the eastern Tianshan, southern part of Central Asian Orogenic Belt (CAOB). Most of these mafic–ultramafic complexes are associated with Cu-Ni-Co deposits. However, Cihai, located in the southern part of the eastern Tianshan, is a large Fe deposit hosted in the Early Permian mafic rocks. The mafic to intermediate rocks are composed of gabbro, diabase, and monzodiorite. Geological and geochemical characteristics suggest that their parental magmas might have been generated by interaction between the depleted asthenospheric mantle and the metasomatized lithospheric mantle. Iron ores of the Cihai iron deposit are hosted in the diabase, and all Fe–Ti oxides in the ore-hosted diabase are ilmenite, instead of magnetite as previously reported. Chondrite-normalized REE patterns show that the magnetite separates from disseminated, banded, and massive iron ores, which are distinct from those in magmatic Fe-Ti deposits. Geological and chemical features suggest that the main ore bodies in the Cihai iron deposit are of hydrothermal origin, rather than magmatic as previously suggested. Numerous other Early Permian mafic rocks were recently identified in the Tarim basin and the eastern Tianshan with ages between 301 and 269 Ma. The mafic rocks in the Tarim basin exhibit characteristics of Oceanic Island Basalt (OIB), whereas the mafic rocks in the eastern Tianshan show island arc basalt (IAB) affinity. In addition, the presence of skarn iron deposit instead of Fe–Ti oxide deposit in the eastern Tianshan during the Early Permian time also lends little support for a plume-related environment. These features, together with a lack of verified anomalous high-temperature magmas in the eastern Tianshan, suggest that the Permian Tarim mantle plume may not account for the formation of the mafic rocks in the eastern Tianshan area, and that the Tarim LIP does not extend to the eastern Tianshan area.
The Almalyk porphyry cluster in the western part of the Central Asian Orogenic Belt is the second largest porphyry region in Asia and hence has attracted considerable attention of the geologists. In this contribution, we report the zircon U–Pb ages, major and trace element geochemistry as well as Sr–Nd isotopic data for the ore-related porphyries of the Sarycheku and Kalmakyr deposits. The zircon U–Pb ages (Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)) of ore-bearing quartz monzonite and granodiorite porphyries from the Kalmakyr deposit are 326.1 ± 3.4 and 315.2 ± 2.8 Ma, and those for the ore-bearing granodiorite porphyries and monzonite dike from the Sarycheku deposit are 337.8 ± 3.1 and 313.2 ± 2.5 Ma, respectively. Together with the previous ages, they confine multi-phase intrusions from 337 to 306 Ma for the Almalyk ore cluster. Geochemically, all samples belong to shoshonitic series and are enriched in large-ion lithophile elements relative to high field strength elements with very low Nb/U weight ratios (0.83–2.56). They show initial (87Sr/86Sr)i ratios of 0.7059–0.7068 for Kalmakyr and 0.7067–0.7072 for Sarycheku and low εNd(t) values of −1.0 to −0.1 for Kalmakyr and −2.3 to 0.2 for Sarycheku, suggesting that the magmas were dominantly derived from a metasomatized mantle wedge modified by slab-derived fluids with the contribution of the continental crust by assimilation-fractional-crystallization process. Compared to the typical porphyry Cu deposits, the ore-bearing porphyries in the Almalyk cluster are shoshonitic instead of the calc-alkaline. Moreover, although the magmatic events were genetically related to a continental arc environment, the ore-bearing porphyries at Sarycheku and Kalmakyr do not show geochemical signatures of typical adakites as reflected in some giant porphyry deposits in the Circum-Pacific Ocean, indicating that slab-melting may not have been involved in their petrogenesis.
Over a hundred iron ore occurrences have been discovered in the Chinese Altay Orogenic Belt (AOB), which is a key Fe-ore belt in NW China. In the Jia'erbasidao ore district, Fe skarn mineralization was mainly developed along the contact zone between biotite granite and the marble of Altay Formation, which is different from most Fe mineralization (non-skarn contact zone) in the Chinese AOB. Two generations of garnet (Grt1and Grt2) have been identified to be related to the two stages of Fe-ore formation at Jia'erbasidao, and Grt1 and Grt2 can be further divided into Grt1a, 1b and Grt2a, 2b, respectively. Our U-Pb dating indicates that the biotite granite and garnet from early skarn stage (Grt1a) were formed at 275.6 ± 3.1 Ma and 270.6 ± 4.4 Ma, respectively. The mineralized biotite granite is metaluminous to peraluminous, LREE-enriched and HREE-depleted. Meanwhile, two Grt2a samples from the overprinting mineralization stage yielded lower intercept 206Pb/238U ages of 248.3 ± 2.9 Ma and 246.0 ± 6.0 Ma, respectively. Grt1 and Grt2 may have formed from different ore-forming fluids: Grt1a was precipitated from a LREE-depleted, near neutral and low fO2 fluid (δEu = 1.92–2.84); Grt1b and magnetite (Mag1) were precipitated from the same fluid with higher fO2 (δEu = mostly 1.22–2.63); Grt2a was precipitated from a fluid with relative LREE enrichment, relatively lower pH and higher fO2 (δEu = 0.98–1.69); Grt2b and magnetite (Mag2) was formed from a fluid that was HREE-enriched, near neutral, and higher fO2 (δEu = 0.76–1.09). Our study shows a two-stage Fe mineralization at Ja'erbasidao, including the Early Permian Fe skarn mineralization and the Early Triassic skarn mineralization.
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